Potting of electrical apparatus



Afiril14,1 959 mSFFEDER I 2,882,505

PO'I 'TING OF ELECTRICAL APPARATUS Filed Juhe 15, 1954 FREE SPACE BETWEEN PARTICLES FILLED W/TH SULFUR HEXAFL UOR/DE PART/CLES 0F FUSED ALUM/NA COATED WITH SILICONE RES/N IN l/E N TOR H. s. FEDEI? A TTOPNE V United States Patent Ofiice 2,882,505 Patented Apr. 14, 1959 POTTING OF ELECTRICAL APPARATUS Herbert S. Feder, Fanwood, N.J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application June 15, 1954, Serial No. 436,950

Claims. (Cl. 336--96) This invention relates to the potting of electrical apparatus. It relates particularly to electrical devices en closed in containers in which particulate potting compositions are used and it relates further to these particulate compositions.

Electrical devices such as coils, transformers, networks or the like are commonly enclosed in metal cans to pro tect them from damaging environments. When any substantial amount of heat is generated by these devices, it is common to fill the free space of the can with a filling or potting material which will afford a better means for conducting heat to the outside of the can than does the free air space.

It is necessary that such a filling or potting compound have an adequate heat conductivity and that it be relatively inert. It is desirable in most instances that it have a good insulation resistance in humid as well as in dry atmospheres. It must not deteriorate on heating and must not cause damage to the apparatus or the can due to expansion on heating. It must also be pourable so that it can be readily introduced into the container and :so that it will fill the small spaces in the apparatus or between the container and the apparatus.

The potting composition of the present invention possesses an extremely desirable combination of these requisite characteristics. It consists of a mass of fine particles of fused alumina, each particle preferably being coated with a cured silicone resin. The manner in which this potting composition is used in the potting of electrical apparatus is illustrated in the accompanying drawing.

This drawing shows a front elevation, in section, of a transformer potted in accordance with the present invention, the potted unit being shown in the position in which it is placed for the filling operation.

In this drawing, a transformer 1 is situated within a metal can made up of a side wall portion 2, a lower cap 3 and an upper cap 4. The transformer is supported by means of a bracket 5 mounted on the lower cap 3. Three transformer leads 6 are respectively electrically connected to terminal leads 7 which pass through insulating sleeves 8 to the outside of the can where they form terminals 9. Sleeves 8 are mounted in cap 3 and serve to form a seal for terminal leads 7 as they pass through the cap. A fourth transformer lead (not shown) is grounded.

The free space between the can and the transformer is filled with a mass 10 of the silicone-coated alumina particles which represent the potting compound of the present invention. This filling may be accomplished either by leaving upper cap 4 off the can until filling is completed or by filling through an orifice in cap 4 which is later sealed.

Although the can may be completely filled to the top with the filling compound 10, the manufacturing procedure is simplified if this filling is carried out to a level somewhat below the top of the can and if a layer of a casting resin is formed on top of the filling compound.

This layer of casting resin is shown as 11 in the drawing. The sealing of the unit is completed by sealing the cap 4 to the remainder of the can in the conventional manner, as by soldering, or if filling has been accomplished through an orifice in the cap, the orifice is sealed.

Prior to the present invention, one of the most satisfactory potting compounds for apparatus of this type had been found to be a mixture of asphalt with a mineral filler such as sand or mica. This normally rigid material was heated to a temperature at which it could be poured, and was then poured hot into the container in which the apparatus to be potted was situated. In heat conductivity, the potting composition of the present invention ap proaches that of the asphalt composition. The composition of the present invention has a very high insulation resistance Which is not appreciably affected by humidity or high temperatures. It is inert and has an indefinite shelf life. It can be exposed to very high temperatures without deterioration and does not either liquefy or expand significantly at high operating temperatures.

Its advantages are several over potting compounds, such as the asphalt base compositions, which are rigid at room temperature and which are made pourable by softening with heat. First, the composition of the present invention is pourable in the cold state so that filling is simplified and made less hazardous. Secondly, it penetrates more readily into small spaces within the container than do the heat-softened materials since these materials tend to become chilled and solidified by the apparatus during the pouring process. For this reason, when the compositions of the present invention are employed, it is possible to use a can which affords less clearance around the apparatus with a consequent saving of both space and weight. Third, since these compositions are not heat sensitive as are the hot-poured materials, there is less limitation upon the operating conditions under which apparatus containing them can be used. A further advantage lies in the fact that the apparatus can be readily unpotted for servicing when the compositions of the present invention are used, a procedure which is not feasible when potting compounds of the asphalt type are used.

The good heat conductivity of the compositions of the present invention is due to the nature of the fused alumina itself. Good pouring characteristics are obtained when fused alumina particles are used which are rotund. The rotund particles may be spherical or substantially spherical, but it is necessary only that they be rounded and free from sharp edges or substantial irregularities. If fused alumina particles are used which are irregular and have sharp edges, the other advantages referred to are obtained but pouring is somewhat more difiicult. The alumina particles are available commercially in different densities due to the presence or absence of entrapped voids or gas bubbles within the particles.

The limitations upon the particle size of the alumina particles are dictated by practical considerations. For apparatus of moderate size it is ordinarily desirable that the particle size should not exceed that which will pass a No. 40 sieve (United States standard screen scale). This sieve has openings of .0166 inch and has 37.9 mesh per inch. If larger particles are used, some difiiculty may be encountered in filling apparatus having small clearances. Preferably particles are used which are not larger than those which will pass a No. 45 sieve (having sieve openings of .0139 inch and having 44.7 mesh per inch). Obviously, however, with larger apparatus and larger clearances, satisfactory results can be obtained with larger particles. A satisfactory fused alumina is a commercially available material having a distributed particle size, the particles of which are rotund and will all pass a No. 45 sieve. This material has a specific gravity of 3 1.19 when packed into, a container and tapped so as to settle the material.

When i116" 55M616 size is substantially below that which is retained upon a No. 200 sieve (having sieve o enin f 2 i h, t 0 p r n h) t e P9 4? i s aractrisfics becom ess esi ab e due o. P in nd m nn s was s des ab e o or th due to the fact that it can be blown by air currents. However, aside from these factors, the fine material forms a satisfactory potting compound. Preferably, no substantial proportion of the material has a particle size sufiiciently small to pass a No. 120 sieve (having sieve. s? s Q 90 nd s Pe n h)- 4 Retention of the v high insulation resistance of the nia s a srin not .nmi cond t n i obtained when the particles are coated with a silicone resin. This resin coating. net only efiiectively resists any undesirable elec-. trical efiect due to moisture, but also is capable of being eir posed to relatively operating temperatures with-, out; (1 ioration of either its physical or electrical char-- wetnesssilicone resin applied to the alumina particles il h i n f dime splu on in a v l le or n vent. satisfactory solution consists of between 1 part and 5 parts of silicone resin per 100 parts of solvent by by volume, A typical solvent which may be used is )gylene Sufficient of the solution is mixed with the alu-. mina powder to wet the powder completely, excess so-. lutionbeing. drained off. The silicone-treated alumina is the r dried air ervacuum to insure that the bulk of: the. solvent is removed. The dried, coated alumina is then halted at a curing temperature until cured; for instance, it is baked; to; at least one hour at a temperature between. ,85. 1 an A. ny silicone resin which is capable of being cured to an ipfusible state. satisfactory. The silicone resins are polyorganosil-oxares in which the most common organic s tituent on t ilicon atoms are alkyl, aryl or aralkyl, rad cals. and which these substituents are present in an onnts greater-tha 1;, but less than 2', per silicon atom. Among more common silicones, are ethyl silicone and: methyiphenyl silicone. A particularly satisfactory silieoneJior the purposes of the present invention is thatsold commercially as Dow Corning silicone DC-996.

When the pott ng of the present invention iscarried: out with alumina particles which are not coated, the sir s-tests disqus rd aboy ar o tain except that the very high insulat'on esistance of the material is adverse-. 1y affected. by liiiindli). In apparatus where this-loss be ngnlnfionresistauw, is not important orwhere the potting material, is, not lilgely to be reached by a humid atmos it, is feasible to use uncoated alumina. r Th? FS- n (high he layer 1 is composed y be casting resin. Barticularly suitable. are the epoxy 7 These resins are formed by the condensation ofi' 5; compounds containing at least two epoxy groups. Pa cularly suitahle are. the. epoxyresins which are di: glycidyl ethersof diphenols and particularlydiglycidyl' ethers of. dihydrosy diphenyl -methanes, such as 4,4 dihyfilgO ydiphenyldimethylmethane. The epoxy resins are ordinarily cured-,by mixing them withv an amine catalyst 2 h Q ig; hen1; to, a curing. temperature. The characteristies, oh the resin are. improved if substantially an eq al amount of any-finely, divided filler such as silica is ,mi; ,ed, with; the resin prior to curing. Curing can be accomplished by balring the completed assembly at a temperature between lf/Sf F. and 195 F. for three holi The dielectric fiakdown strength of the mass 3 of q npqupd ca desired; be improved by re;

placing the air in the spaces between the particles with a high dielectric, streng h gas, such as ulp ur hexafluoride. under either atmospheric or superatmospheric pressure.

The invention has been described above in terms of specific embodiments and, since certain modifications and equivalents may be apparent to those skilled in the art, this description is intended to be illustrative of, but not necessarily to constitute a limitation upon, the scope of the invention.

What is claimed is:

1. An encased electrical device comprising a container, an electrical device within said container and a potting material in the space between said device and the walls of said container, said potting material consisting of round, discrete particles of fused alumina coated with a cured organosiloxane having more than one but less than two organic substituents per silicon atom, which discrete coated particles are, when unconfined, freely flowable one. over the other, the interstices between said particles being filled with a gaseous medium.

2. A device as described in claim 1 wherein the free space between the particles of potting material is filled with sulphur hexafiuoride.

3:. An encased electrical device comprising a sealed container, an electrical coil within said container and a potting material in the free space within said container, said plotting material consisting of round, discrete particles of fused alumina having a particle size between about .0l66. inch and about .0029 inch, each of said particles being coated with a cured organosiloxane having more than one but less than two organic substituents per silicon atom, which discrete coated particles are, when unconfined, freely/ flowable one over the other, the interstices between said particles being filled with a gaseous medium,

4-. A transformer enclosed in a sealed container filled with a potting composition consisting of discrete, rounded, fine particles of fused alumina having a particle sizenot exceeding about .0166 inch, each of said particles being coated with a cured organosiloxane having more than one but less than two. organic substituents persilicon atom, which. discrete. coated. particles are, when unconfined, freely flowable one over the other, the interstices between said particles being filled with a gaseous medium.

5-. Any encased electrical device comprising a container, an electrical device. within said container and a potting material in. the space between said device and the walls of said container, said pot-ting material consisting of discrete, fine particles comprising fused alumina, coated with a cured organosiloxane having more than one but less. than two. organic substituents per silicon atom, which discrete coated particles are, when unconfined, freely fiowable one over the other, the interstices between said particles being filled with a gaseous medium.

References Cited in. the file of this patent UNITED STATES PATENTS 

